Producers of metal strips are using cold roll processes for producing a metal strip with specified mechanical properties, surface properties and thickness. In the cold rolling process, the strip passes through a nip or roll gap existing between two counter-rotating rolls for reducing the thickness of the strip and providing the required surface quality. During the cold roll process a lot of heat is created in the nip due to the friction between the rolls and the strip and due to the deformation of the strip material. This heat has negative influences on the material and surface properties.
In conventional cold rolling processes, liquids, such as oil, water or emulsions, are used as a cooling lubricant for reducing the friction and the heat in the roll gap. Often, these liquids remain on the surface after the cold rolling where they cause negative effects. E.g. water or aqueous emulsions on the surface of the metal strip lead to corrosion, i.e. rust formation. Further, oil residues on the surface have to be removed therefrom as far as possible prior to further processing of the metal strip. Both, the cleaning process and the rejects due to aqueous or oily residues on the surface of the metal strip cause high costs in rework and scrap.
Accordingly, it is an object of the present invention to provide a process and a roll stand for cold rolling of a metal strip, wherein the above-mentioned problems arising from residues on the surface of the metal strip are eliminated to a large extent. According to a second aspect of the present invention a process and a roll stand for cold rolling of a metal strip is to be provided where the deposition of ice or water within the roll stand and/or on the surface of the metal strip to be processed is avoided to a large extent.
According to the present invention there is provided a process for cold rolling of a metal strip, wherein the metal strip passes through a nip between two counter-rotating rolls, driven in counter-rotation substantially at room temperature, wherein a cold and/or liquefied gas, preferably an inert gas, is blown into the area of the nip or roll gap. A roll stand according to the present invention comprises two counter-rotating rolls forming a nip or rolling gap and nozzle means for blowing a cold and/or liquefied gas, preferably an inert gas, through at least one orifice of said nozzle means into the area of the roll nip. Preferably, the temperature of the cold and/or liquefied gas is appreciably lower than room temperature. The term xe2x80x9ccold and/or liquefied gasxe2x80x9d as used herein relates to a cold fluid in the gaseous or liquid phase or in a phase mixture of gas and liquid.
According to the invention the gas acts and as a cooling agent for cooling the metal strip during the cold rolling process and apparently as a lubricant for reducing friction between the rolls and the metal strip. The cooling effect is stronger if the gas is applied as a liquefied gas due to the larger specific heat of a liquid. According to the invention, the cooling agent, i.e. the gas, vaporizes during and after the cold rolling process without residuals on the surface of the metal strip. Accordingly, the present invention has the advantage that the cooling agent does not have to be removed in a separate process step after the cold rolling process. According to the present invention the gas creates a protective layer between the strip and the rolls. Preferably, the gas is an inert gas thereby avoiding oxidation of the surface of the metal strip.
Due to the better cooling and apparent lubrication effect according to the present invention the metal strip virtually is free of cracks and pores and also the surface quality is better and more uniform. In particular, matte areas that cover the surface of the processed metal strip more or less completely in the conventional cold rolling process using a liquid lubricant are avoided according to the present invention.
The nozzle means according to the invention preferably comprises a plurality of nozzles or orifices for blowing the cold and/or liquefied gas into the region of the nip that are arranged at regular intervals over the width of the metal strip. Preferably, the nozzles or orifices are positioned upstream of the roll nip. The nozzles or orifices may be positioned above and/or below the metal strip. The cold and/or liquefied gas may be blown into the area of the roll nip perpendicular to the metal strip or substantially tangential to the surface of the rolls.
The inventors have observed that two new different types of surface defects occur, when a very cold gas, e.g. liquefied nitrogen gas is used. Namely, oval long matte areas and small matte points have been observed on the surface of the metal strip after the cold rolling process. The inventors have found out that some of these defects can be attributed to the creation of frozen atmospheric water vapor around the nozzles as well as around the feed line to the nozzles and to the water resulting from condensed atmospheric water vapor. Some of the defects observed could also be attributed to drops of liquefied gas, e.g. of liquefied nitrogen gas, falling onto the surface of the metal strip to be processed.
In order to avoid these problems, the process for cold rolling of a metal strip according to the present invention may further comprise a step of shrouding or shielding the nozzle means at least near the orifice of the nozzle means from the ambient atmosphere for preventing the creation of water or ice near the orifice of the nozzle means due to frozen or condensed atmospheric water vapor. Accordingly, the creation of matte areas on the surface of the metal strip can be avoided.
According to a first embodiment of the present invention the jets of cold and/or liquefied gas and/or the orifices of the nozzle system are shrouded or shielded by a flow a dry gas around the jet and/or the orifices during the cold rolling process. Thus, it can be avoided that water vapor from the ambient atmosphere enters the cooled region, e.g. the roll nip with the metal strip there between and/or the orifices of the nozzle system. Thus, the condensation or crystallization of the water vapor is eliminated.
In principle any pure gas, i.e. not containing agents that could condense or crystallize to thereby cause the above-mentioned matte areas or surface defects, can be used according to the present invention. Preferably, the dry gas should be an inert gas. The process and roll stand according to the present invention may be simplified further, if the flow of dry gas is branched off from the flow of cold and/or liquefied gas, which flows to the orifices of the nozzle means and is used for cooling.
The dry gas may be applied as a curtain of dry gas surrounding the jets of cold and/or liquefied gas emitted from the orifices of the nozzle means. Preferably, this curtain of dry gas shrouds the entire area both of the orifices of the nozzle means and of the roll nip including the metal strip being cooled by the cold and/or liquefied gas.
Preferably, each feed line of an orifice for supplying the orifice of the nozzle system with the cold and/or liquefied gas is surrounded by a tube or a box-shaped structure through which the dry gas is blown towards the metal strip. Thus, the flow of dry gas is guided to flow substantially in parallel to the jet of cold and/or liquefied gas. Thus, it can be avoided that condensed water vapor or ice crystals from the ambient atmosphere fall onto the surface of the metal strip, where they would cause defects. A further advantage is that the amount of dry gas required for shrouding the orifices and/or jets of gas may be reduced substantially. A further advantage is that due to the steady flow of dry gas around the orifices of the nozzle system any deposition of ice or water on the orifices can be prevented completely.
Preferably, the jets of cold and/or liquefied gas are emitted from the orifices of the nozzle means in the shape of a cone with the center in the middle of the respective orifice. Thus, a uniform distribution of gas in the area of the roll nip can be ensured. For a better shrouding the orifice may be located within the tube or box-shaped structure at a distance to the front face of the tube or box-shaped structure so that the cone does not intersect the tube or box-shaped structure on its way towards the metal strip.
If the liquefied gas is fed to the orifices of the nozzle means, a part of the liquefied gas normally vaporizes. The gas bubbles thus created in the feed line causes pressure differences at the orifices or nozzle outlets and thus a pulsation of the gas jet emitted and of the liquefied gas supply. This pulsation is even amplified further within the feed line, because the gas of the bubbles has a smaller specific heat resulting in a less efficient cooling at certain regions within the feed line for liquefied gas. The pulsation of gas causes a non-uniform cooling effect in the area of the roll nip and may also dislodge ice crystals near the orifices or the nozzle means. Furthermore, the pulsation of gas in the feed line might also cause mechanical vibrations of the feed line that might also dislodge ice crystals near the orifices or the nozzle means or on the surface of the feed line. The inventors have observed, that these pulsation contribute to long oval matte areas on the surface of a metal strip.
For the purpose of eliminating these problems, the dry gas flowing through the tube or box-shaped structure surrounding every feed line of the nozzle means is preferably derived directly from the flow of cold and/or liquefied gas for cooling. Thus, the exterior of the feed line and the orifices of the nozzle means can be cooled efficiently thereby reducing the above-mentioned two-phase flow of gas in the feed line.
Preferably, the flow of gas through the tube or box-shaped structure is regulated by a control valve in order to obtain a constant cooling rate and a constant shrouding effect. Preferably this control valve is used simultaneously as a throttling means for expanding the cold and/or liquefied gas to thereby reduce its temperature.
Thus, the temperature of the gas flowing through the tube or box-shaped structure may be lowered below the temperature of the gas in the feed line to thereby further eliminate the above-mentioned two-phase flow. Thus sub-cooling of the feed line can be achieved in an efficient manner.
In order to further avoid the condensation or crystallization of water vapor from the ambient atmosphere heat exchange means or other heating means may be provided, preferably at the front end of the nozzle means. The heat exchange means may surround the tube or box-shaped structure, preferably only at a front portion. The fluid may flow through the heat exchange means.
According to a second embodiment of the present invention a shrouding at least near the orifices of the nozzle means from ambient atmosphere is provided by a suitable mechanical structure for preventing the creation of condensed water or ice stemming from atmospheric water vapor near the orifices or nozzle outlets.
According to this second embodiment shrouding may be provided by any mechanical structure sufficiently shielding the orifices or the nozzle means and/or the feed lines from ambient atmosphere. Such a shrouding may be provided by a single box surrounding all orifices or nozzle outlets and at least a portion of their respective feed lines for supplying cold and/or liquefied gas. Preferably, such a box has a front cover with openings aligned with the respective orifices to allow the flow of cold and/or liquefied gas towards the metal strip. Instead of a single box also a plurality of boxes may be provided, each for a respective orifice of the nozzle means. As an alternative, a tube may surround each orifice or nozzle outlet and at least a portion of the associated feed line. Thus, the orifices can be shrouded in a simple and cost efficient manner.
The second embodiment of the present invention may be preferred, if a cold and/or liquefied gas at a moderate temperature as compared to room temperature is used for cooling, because at moderate temperatures the condensation and crystallization of atmospheric water vapor is used. An example of a liquefied gas used according to this second embodiment is carbon dioxide gas. This may be sufficient, e.g. for roll stands not used in continuous operation or with a relatively low throughput.
Hereinafter, specific examples of preferred embodiments according to the present invention will be described. When read with reference to the Figures, further advantages, features and objects of the present invention will become aware to the skilled artisian.